In many fiber optic applications, it is desirable to have a switch which in one switching state couples light emanating from a first optical fiber into a second optical fiber and which in another switching state couples light from the first optical fiber into a third optical fiber.
Illustratively, such a switch is useful in a fiber optic local area network implemented in a ring like configuration. Such a network comprises a plurality of nodes arranged in a ring, the nodes being interconnected by optical fibers. To transmit information from an originating node to a destination node by way of the ring like optical fiber network the information is transmitted from the originating node along the ring from one intervening node to the next until the destination node is reached. Illustratively, information arrives at a node via an incoming optical fiber. The incoming optical fiber is coupled to a receiver which converts the incoming information signal from optical to electrical form. Information leaves a node via an optical transmitter which converts a signal from electrical to optical form, the optical signal being transmitted out from the node via an outgoing optical fiber.
One problem with a fiber optic local area network having a ring configuration is that if one node fails the entire network or a significant portion thereof will be nonusable as no information can pass through a failed node. Accordingly, it is desirable to utilize an optical switch which has two switching states. In the first switching state light from an incoming fiber is coupled to a node receiver and light from the node transmitter is coupled to an outgoing fiber so that the node is inserted into the network. In the second switching state, the incoming and outgoing fibers are coupled so that the node is bypassed. When a node is bypassed in this manner, the network can continue to function despite a node failure.
In the event of a node failure or suspected node failure, a node should be tested by enabling the transmitter in the node to communicate via an optical path with the receiver in the node. Accordingly, it is desirable for the aforementioned optical switch, when in its second state, to optically couple the transmitter of the node to the receiver to enable the node to e tested.
An example of such an optical switch is disclosed in U.S. patent application Ser. No. 053,220, entitled "Fiber Optic Bypass Switch" filed on May 13, 1987 and assigned to Kaptron Inc. The contents of this application are incorporated herein by reference.
The optical switch disclosed in the above identified patent application is based on an imaging property of a spherical reflecting surface. In particular a point source of light slightly displaced from the center of curvature of the spherical reflector is imaged with minimal aberration at a point symmetrically located with respect to the center of curvature. The switch of the above identified patent application involves rotating a spherical reflector between first and second positions relative to an array of optical fiber end faces. Illustratively, there is an optical fiber end face (T) which terminates a fiber connected to the transmitter of a node and an optical fiber end face (R) which terminates a fiber connected to the receiver of the node. Two additional fiber end faces (I and 0) terminate the incoming fiber and the outgoing fiber respectively. There are also two other optical fiber end faces (L1 and L2) which correspond to the first and second ends of a fiber optic loop. Instead of utilizing fibers connected to the transmitter and receiver, these devices themselves may form part of the array.
In the first reflector position, the T and 0 end faces are conjugate (i.e. symmetrically located with respect to the center of curvature) and the I and R end faces are conjugate. Accordingly, light emanating from the input fiber end face (I) is imaged by the spherical reflector into the receiver fiber end face (R) and light emanating from the transmitter fiber end face (T) is imaged into the output fiber end face (0). Thus in its first position the switch of the present invention can be used to insert a node into a fiber optic network by coupling light from the incoming fiber to the receiver and light from the transmitter into the outgoing fiber.
In the second reflector position, the input and output fiber end faces (I and 0) have conjugate locations so that light from the incoming fiber is now imaged by the spherical reflector into the outgoing fiber instead of the receiver so that the node is bypassed. Simultaneously, in the second reflector position the transmitter fiber end face (T) and the first fiber loop end face (L1) are conjugate and the receiver fiber end face (R) and the second fiber loop end face (L2) are conjugate. Thus, light emanating from the transmitter fiber end face (T) is imaged by the reflector into the first fiber loop end face (L1). This light enters the fiber loop and emerges at the second fiber loop end face (L2). Light emanating from the second fiber loop end face (L2) is imaged by the reflector into the receiver fiber end face (R). Thus the transmitter and receiver are connected to each other by an optical path enabling the bypassed node to be tested. The fiber optic loop used in the testing path between the transmitter and receiver provides a suitable amount of attenuation so that the receiver is not saturated when the node is tested.
The operation of the switch of the above identified patent application may be summarized as follows. Light arrives via a first optical fiber (e.g. the incoming optical fiber) and may be imaged by a spherical reflector into a second optical fiber chosen from a plurality of available fibers (e.g. the receiver fiber or the outgoing fiber) depending on which of two positions the reflector is pivoted into.
One shortcoming of the switch described in the above identified patent application is that it depends for operation on pivoting a spherical mirror from a first position to a second position in order to switch from a first switching state to a second switching state. Such a pivoting design may not be sufficiently rugged and environmentally stable for certain network applications. In addition, the pivoting movement of the mirror may in certain applications impose an unacceptably long switching time between the first and second switching states.
Accordingly, it is an object of the present invention to provide an optical switch which relies on the symmetry of an imaging system to image light arriving via a first optical fiber into a second optical fiber when the switch is in one state and into a third fiber when the switch is in another (i.e. second) state, but which switch does not rely on a pivoting movement of a spherical reflector to switch between the two states.
It is a further object of the invention to provide a switch based on the symmetry of an imaging system, which switch in a first state couples radiation from an incoming optical fiber to the receiver of a node and radiation from the transmitter of the node to an outgoing fiber so that the node is inserted in a network, and which switch in a second switching state couples the incoming and outgoing fibers so that the node is bypassed and provides a path between the node transmitter and receiver so that the node may be tested.